1. Field of the Invention
The present invention is related generally to load lock controls for vacuum processing chambers and more particularly to a combination differential and absolute pressure transducer for load lock control and a method of controlling load locks with such combination differential and absolute pressure transducer.
2. State of the Prior Art
Vacuum processing in reaction chambers is commonly used to deposit thin films of semiconductor materials, metal, dielectrics, and the like onto substrates in the fabrication of semiconductor devices. Typical processes that utilize such vacuum reaction chambers include chemical vapor deposition (CVD) and physical vapor deposition (PVD) and many variations of such processes, as well as etching processes to clean substrates or remove selected portions of materials. Typically, the vacuum process chamber is evacuated with a vacuum pump to a very low pressure, for example down to 10−4 torr, and, in some processes, much lower, such as 10−6 or even 10−7 torr. When the desired vacuum is attained, feed gases are flowed into the process chamber at desired rates and proportions to react and/or deposit desired materials onto substrate wafers. Heat may be used in some processes, but others are performed at room temperature. When deposition of the desired materials is complete, the wafer is removed from the process chamber and another substrate wafer is inserted into the process chamber, where the deposition process is repeated.
Significant vacuum pumping time is required to pump the process chamber down to the desired pressure, and undesirable contaminants enter the process chamber every time it is opened to atmosphere. Therefore, substantial efforts are made to avoid opening the process chamber to atmosphere and to maintain the process chamber pressure as close to the desired low deposition pressure as possible. Load locks are used, therefore, to facilitate insertion of substrates into the process chambers for deposition and/or etch processing and to remove the wafers from the process chamber while maintaining the vacuum in the process chamber.
A load lock is, essentially, a second vacuum chamber, often smaller in size than the process chamber, and connected to the process chamber by a passage with an interior “door” or large valve that can be opened for insertion and removal of the wafers into and out of the process chamber. When the interior door is closed, it seals the passage so that no air or gas can flow into or out of the process chamber through the passage. The load lock also has an exterior “door” or large valve, which opens the load lock chamber to the atmosphere to allow insertion or removal of wafers into and out of the load lock chamber. When the exterior door is closed, it seals the load lock so that no air or other gas can flow into or out of the load lock chamber.
In operation, the process chamber has its pressure maintained at the desired vacuum by a process chamber vacuum pump. With the interior door of the load lock closed, the exterior door is opened to the atmosphere, so one or more wafer substrate(s) can be inserted into the load lock chamber. With the wafer(s) in the load lock chamber, the exterior door is closed, and a load lock vacuum pump draws the air out of the load lock chamber, until the pressure in the load lock chamber is about as low as the pressure in the process chamber. Then, the interior door is opened, so the wafer substrate(s) can be moved from the load lock chamber, through the passage, and into the process chamber. When the wafer(s) are in the process chamber, the interior door can be closed while the wafer(s) are processed in the process chamber, i.e., while feed gas is fed into the process chamber and materials are either deposited on, or etched from, the wafer(s). Alternatively, but not preferably, the interior door could be left open during processing.
When the processing is complete, the wafer(s) are removed from the process chamber into the load lock chamber. The interior door is then closed to maintain the vacuum in the process chamber, while the pressure in the load lock is brought up to atmospheric pressure by allowing air or an inert gas, such as nitrogen, to flow into the load lock chamber. When the pressure in the load lock chamber is at or near atmospheric pressure, the exterior door is opened to allow removal of the processed wafer(s).
Some more complex process systems have a central transfer chamber with several process chambers branching out from the transfer chamber. In those circumstances, the load lock is usually connected by the passage and interior door to the transfer chamber.
In the past, it has been difficult to control the load lock in an efficient manner. Convection pirani pressure sensors, which have absolute pressure measuring capabilities from about 1,000 torr down to about 10−3 torr (atmospheric pressure at sea level is about 760 torr) have been used in pressure transducers adapted to control opening of the doors in load locks. Such control of load lock doors with that type of pressure transducer has been beneficial, but problems persist. For example, the 10−3 torr lower pressure measuring limit of the convection pirani sensors is not low enough for effective control of opening the interior door, because the process chambers are usually operated at pressures at least one to three orders of magnitude below that limit, i.e., at 10−4 torr or even 10−6 torr or lower. Thus, even when the load lock pressure is pumped down to 10−3 torr, opening the interior door causes an undesirable rush of gas molecules, along with any particulate impurities and water vapor they carry along, into the process chamber. It puts a greater load on the vacuum pumps of the process and/or load lock chambers, causing larger pump down times after each opening and closing of the interior door, especially in the process chamber to get the pressure pumped back down to the desired process pressure. Such added pumping overhead adds to the processing time and decreases efficiency.
The problems are even worse on the upper pressure end, i.e., at or near atmospheric pressure (about 760 torr), because density of gas or air molecules is much greater at that pressure than at the vacuum pressures used in vacuum process chambers. Thus, opening the exterior door when pressure inside the load lock chamber is not the same as the ambient atmospheric pressure causes much stronger air currents and is much more contaminating, even when the load lock is in a clean room. Again, convection pirani sensors do have accurate pressure sensing capabilities in the atmospheric range, but it is impossible to set them to control exterior door opening effectively due to constantly changing ambient atmospheric pressure conditions due to weather, altitude, and the like. For example, some manufacturers set the transducer to generate a signal to open the exterior door of the load lock when pressure of the load lock chamber is brought up to 750 torr, thinking it will work for most locations that are slightly above sea level. However, ambient atmospheric pressure in Boulder, Colorado, for example, is about 630 torr, so having a transducer that opens the exterior door when pressure in the load lock chamber reaches 750 torr in Boulder, Colo., would still have adverse gas current and contamination effects. Further, ambient atmospheric pressure at any geographic location varies, such as with different weather conditions or fronts that move into and out of any particular location. Resetting such transducers to generate control signals at different pressures is not easy, may require changing software or control circuits, and is not something that is done by ordinary users.
Therefore, there is a need for better transducer apparatus and for better control methods for controlling the exterior door openings, especially, and also for controlling interior door openings in load locks.